1. Field of the Invention
The present invention relates to a resin composition for semiconductor encapsulation and a semiconductor device using the same.
2. Description of the Related Art
In the recent trend of down-sizing, lighter weight and high performance of electronic devices on the market, high integration of a semiconductor element (also referred to hereinafter as chip) is advancing year by year, and as surface mounting of a semiconductor device (also referred to hereinafter as package) is promoted, the requirements for an epoxy resin composition for semiconductor encapsulation (also referred to hereinafter as encapsulating material or encapsulating compound) become increasingly strict. As semiconductor devices are down-sized and thinned, further high flowability and high strength are required of the epoxy resin composition for semiconductor encapsulation. Especially under the present circumstance where the surface mounting of a semiconductor device comes to be general, a semiconductor device having absorbed moisture is exposed to high temperatures at the time of soldering, upon which the explosive stress of gasified water vapor causes cracking in the semiconductor device, or generates delamination between a semiconductor element or a lead frame and a cured product of the epoxy resin composition, resulting in significant deterioration in electrical reliability. For a tendency toward abolition of use of lead, lead-free solder having a higher melting point than conventional is increasingly frequently used. Because use of the lead-free solder needs the mounting temperature to be higher by about 20° C. than conventional, there arises a problem of more significant deterioration after mounting in the reliability of the semiconductor device than before. Accordingly, prevention of the deterioration in reliability described above, that is, improvement of soldering resistance is a great issue. From the viewpoint of an environmental problem, there is also an increasing need for flame retardancy without using a flame retardant such as a Br compound or antimony oxide. For improving soldering resistance and flame resistance, higher loading of inorganic fillers is regarded effective because water absorption can be reduced for soldering resistance and the content of easily flammable resin can be reduced for flame resistance. Under these circumstances, there is a recent trend for the epoxy resin composition to use a resin of lower viscosity and to load a larger amount of inorganic fillers.
For maintaining high flowability and low viscosity during molding, use of a resin of low melt viscosity is known (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 7-130919 (pages 2 to 10)). For increasing the amount of inorganic fillers loaded, surface treatment of the inorganic fillers with a silane coupling agent is known (see, for example, JP-A No. 8-20673 (pages 2 to 6)). With only these methods given, however, high flowability, soldering resistance and flame retardancy during molding cannot be simultaneously satisfied.
Accordingly, the present applicant have proposed an epoxy resin composition excellent in soldering resistance and flame resistance by using a phenol aralkyl type epoxy resin containing biphenylene structure and a phenol aralkyl type phenol resin containing biphenylene structure (see, for example, JP-A No. 11-140277 (pages 2 to 11)).
However, further improvements in soldering resistance and flame resistance are desired for the recent higher need described above.
In the meanwhile, in the recent trend of down-sizing, lighter weight and high performance of electronic devices on the market, high integration of a semiconductor is advancing year by year, and as surface mounting of a semiconductor package is promoted, an area surface-mounted package is newly developed and is being substituted for a package of conventional structure.
A ball grid array (referred to hereinafter as BGA), or a chip size package (referred to hereinafter as CSP) further pursuing down-sizing, is representative of the area surface-mounted semiconductor package, and these were developed for coping with the requirements for multi-pin and high speed which are in nearly maximum degrees in the conventional surface-mounted package represented by QFP and SOP. In its structure, a semiconductor element is mounted on one side of a rigid circuit substrate represented by bismaleimide/triazine (referred to hereinafter as BT) resin/copper foil circuit substrate, or a flexible circuit substrate represented by polyimide resin film/copper foil circuit substrate, and its element-mounted surface, that is, only one side of the substrate is molded and encapsulated with a resin composition or the like. Solder balls are arranged two-dimensionally in parallel and formed on the side opposite to the element-mounted surface of the substrate, so as to join to the package-mounted circuit substrate. As the element-mounted substrate, a structure using a metallic substrate such as a lead frame other than the organic circuit substrate is also devised.
The structure of the area surface-mounted semiconductor package has a structure encapsulated in one side wherein the substrate is encapsulated with a resin composition in the element-mounted side but not in the solder ball-formed side. In a very few cases, an encapsulating resin layer of about several tens μm is present in the solder ball-formed side of a metal substrate such as a lead frame, while an encapsulating resin layer of about several hundreds μm to several mm is formed on the element-mounted side, and thus the substrate is encapsulated in substantially one side. It follows that due to the imbalance in thermal expansion and thermal shrinkage between the organic or metallic substrate and a cured product of the resin composition or due to the influence of curing shrinkage during molding and curing of the resin composition, these packages are liable to cause warpage just after molding. When the circuit substrate on which these package was mounted is soldered, it is subjected to a step of heating to 200° C. or more, at which the warpage of the package is generated, and a large number of solder balls do not become flat and are raised from the package-mounted circuit substrate, thus also causing a problem of deterioration in electrical connection reliability.
To reduce the warpage of the package wherein only one side of the substrate was substantially encapsulated with the resin composition, a method of reducing the curing shrinkage of the resin composition is known. The organic substrate frequently uses a resin of high glass transition temperature (referred to hereinafter as Tg) such as BT resin and polyimide resin, and these resins have higher Tg than about 170° C. that is the molding temperature of the resin composition. In a cooling process of from molding temperature to room temperature in this case, the organic substrate is shrunk when the linear expansion coefficient of the substrate is in only the region of α1. Accordingly, if the resin composition also has high Tg, its α1 is the same as that of the circuit substrate, and the curing shrinkage is zero, then the warpage is considered to become nearly zero. Accordingly, a method of reducing the curing shrinkage of a resin composition by increasing Tg by combining a triphenol methane type epoxy resin with a triphenol methane type phenol resin has already been proposed (see, for example, JP-A No. 11-147940 (pages 2 to 7)).
For reducing warpage, there is known a method wherein the linear expansion coefficient of a substrate and the linear expansion coefficient of a cured product of a resin composition are made nearly equal to each other. As described above, a method wherein a resin of low melt viscosity is used to increase the content of inorganic fillers thereby adjusting α1 to that of a substrate has already been proposed (see, for example, JP-A No. 11-1541 (pages 2 to 5)). Further, use of a resin containing a naphthalene ring structure estimated to be capable of reducing linear expansion coefficient has also been proposed (see, for example, JP-A No. 2001-233936 (pages 2 to 8)). Such resin composition hardly causes warpage, but due to high water absorption of its cured product, cracking easily occurs during soldering. Further, the resin composition is poor in flame resistance for practical use unless a flame retardant is used. From the foregoing, there has been demand for techniques capable of satisfying low warpage, soldering resistance and flame resistance without deteriorating flowability and curing properties, even in the area surface-mounted semiconductor package.
As described above, there has been a desire for the development of an encapsulating material wherein high levels of soldering resistance, flame resistance and flowability required of both a peripheral surface-mounted semiconductor package with lead frame and a one side encapsulated area surface-mounted semiconductor package are satisfied and simultaneously sufficient low warpage is achieved.